This invention relates to low VOC (volatile organic component) curable coating compositions and more particularly relates to high solids coating compositions having low application viscosity, which are particularly suited for automotive finishes.
As the amount of the VOC from solvent based coating compositions permitted to be released in the atmosphere continues to drop, there is a continuing need for reducing the VOC content of solvent based coating compositions without attenuating their performance characteristics or the ease with which the coatings from these compositions can be applied over substrates. A number of approaches have been tried, one being to increase the solids content of the coating compositions without affecting the performance characteristics of the resultant coating, particularly the mar-resistance and environmental etch resistance.
One such approach, described in PCT Publication No. WO97/44402, is directed to a low VOC coating composition having a linear or branched cycloaliphatic moiety-containing oligomers which, upon cure, form a three-dimensional network having chains of substantially uniform, and controllable molecular weight between the crosslinks. The functionalized oligomers have weight average molecular weights not exceeding 3,000 and a polydispersity not exceeding 1.5. However, a need still exists for low VOC, high performance coating compositions that not only cure under ambient conditions or at elevated temperatures but are still easy to apply using conventional application processes, such as spray coating. The present invention solves the problem by reducing the application viscosity at high solids level without adversely affecting the performance characteristics of the resultant coating.
The present invention is directed to a curable coating composition comprising a binder, which comprises:
a silicon/hydroxyl component and a crosslinking component, said silicon/hydroxyl component comprising:
(I). A silicon/hydroxyl reactive oligomer having a linear or branched cycloaliphatic moiety and at least two functional groups with at least one of said groups being a silane or a silicate, the remaining groups being hydroxyl groups;
(II). A silicon reactive oligomer having a linear or branched cycloaliphatic moiety and at least two functional groups being a silane, silicate or a combination thereof, and a hydroxy acrylic polymer, a hydroxy polyester, a silicon free reactive oligomer having a linear or branched cycloaliphatic moiety and at least two hydroxyl groups, or a combination thereof; or
(III). A combination of said (I) and (II),
wherein said silicon/hydroxyl reactive oligomer, said silicon reactive oligomer and said silicon free reactive oligomer all having a GPC weight average molecular weight not exceeding 4,000 and a polydispersity not exceeding about 1.7; and
said crosslinking component comprising a blocked crosslinker or an unblocked crosslinker wherein said blocked or unblocked crosslinkers being provided with at least two isocyanate groups and wherein the ratio of equivalents of isocyanate per equivalent of hydroxyl groups is in the range of from 0.3/1 to 2.0/1.
One of the advantages of the coating composition of the present invention is its significantly low VOC content even at significantly high solids level.
The coating composition of the present invention advantageously provides for a highly crosslinked system at significantly low application viscosities.
Another advantage of the coating composition of the present invention is that it produces coatings having high performance characteristics, such as mar and etch resistance even at high gloss.
As defined herein:
xe2x80x9cTwo-pack coating compositionxe2x80x9d means a thermoset coating composition comprising two components stored in separate containers. These containers are typically sealed to increase the shelf life of the components of the coating composition. The components are mixed prior to use to form a pot mix. The pot mix has a limited potlife typically of minutes (15 minutes to 45 minutes) to a few hours (4 hours to 6 hours). The pot mix is applied as a layer of desired thickness on a substrate surface, such as an autobody. After application, the layer is cured under ambient conditions or cure-baked at elevated temperatures to form a coating on the substrate surface having desired coating properties, such as high gloss, mar-resistance and resistance to environmental etching.
xe2x80x9cOne-pack coating compositionxe2x80x9d means a thermoset coating composition comprising two components that are stored in the same container. However, the crosslinker component is blocked to prevent premature crosslinking. After the application of the one-pack coating composition on a substrate, the layer is exposed to elevated temperatures to unmask the blocked crosslinker. Thereafter, the layer is bake-cured at elevated temperatures to form a coating on the substrate surface having desired coating properties, such as high gloss, mar-resistance and resistance to environmental etching.
xe2x80x9cLow VOC coating compositionxe2x80x9d means a coating composition that includes less then 0.6 kilograms of organic solvent per liter (5 pounds per gallon) of the composition, as determined under the procedure provided in ASTM D3960.
xe2x80x9cHigh solids compositionxe2x80x9d means a coating composition having solid component of above 40 percent, preferably in the range of from 45 to 87 percent and more preferably in the range of from 55 to 80 percent, all in weight percentages based on the total weight of the composition.
xe2x80x9cGPC weight average molecular weightxe2x80x9d means a weight average molecular weight measured by utilizing gel permeation chromatography (GPC). A high performance liquid chromatograph (HPLC) supplied by Hewlett-Packard, Palo Alto, Calif. was used. Unless stated otherwise, the liquid phase used was tetrahydrofuran and the standard was polymethyl methacrylate.
xe2x80x9cPolydispersityxe2x80x9d means GPC weight average molecular weight divided by GPC number average molecular weight.
xe2x80x9c(Meth)acrylatexe2x80x9d means acrylate and methacrylate.
xe2x80x9cPolymer particle sizexe2x80x9d means the diameter of the polymer particles measured by using a Brookhaven Model BI-90 Particle Sizer supplied by Brookhaven Instruments Corporation, Holtsville, N.Y. The sizer employs a quasi-elastic light scattering technique to measure the size of-the polymer particles. The intensity of the scattering is a function of particle size. The diameter based on an intensity weighted average is used. This technique is described in Chapter 3, pages 48-61, entitled Uses and Abuses of Photon Correlation Spectroscopy in Particle Sizing by Weiner et al. in 1987 edition of American Chemical Society Symposium series.
xe2x80x9cPolymer solidsxe2x80x9d or xe2x80x9cBinder solidsxe2x80x9d means a polymer or binder in its dry state.
xe2x80x9cSilanesxe2x80x9d means the silicon compounds having the Sixe2x80x94C bond.
xe2x80x9cSilicatesxe2x80x9d means the silicon compounds having the Sixe2x80x94Oxe2x80x94C bond.
The present invention is directed to a low VOC curable coating composition that is particularly suited for use in automotive refinishing and OEM (original equipment manufacturer) process. The composition includes a binder in an organic solvent. The amount of organic solvent used results in the composition having a VOC of less than 0.6 kilogram (5 pounds per gallon), preferably in the range of 0.1 kilogram to 0.53 kilogram (1 pound to 4.4 pounds per gallon) and more preferably in the range of 0.1 kilogram to 0.4 kilogram (1 pound to 3 pounds per gallon) of an organic solvent per liter of the composition.
The binder includes a silicon/hydroxyl component and a crosslinking component. The silicon/hydroxyl component includes in the range of from 2 weight percent to 100 weight percent, preferably in the range of from 10 weight percent to 90 weight percent, more preferably in the range of from 20 weight percent to 80 weight percent and most preferably in the range of from 30 weight percent to 50 weight percent of the following:
I. A silicon/hydroxyl reactive oligomer having a linear or branched cycloaliphatic moiety and at least two functional groups. At least one of the groups is a silane or a silicate and the remaining groups are hydroxyl groups.
II. A silicon reactive oligomer having a linear or branched cycloaliphatic moiety and at least two functional groups being a silane, silicate or a combination thereof. The silicon reactive oligomer is blended with a hydroxy acrylic polymer, a hydroxy polyester, a silicon free reactive oligomer having a linear or branched cycloaliphatic moiety and at least two hydroxyl groups, or a combination thereof.
III. A combination of the aforedescribed I and II.
The silicon/hydroxyl reactive oligomer, silicon reactive oligomer and the silicon free reactive oligomer are all provided with a GPC weight average molecular weight not exceeding 4000, preferably in the range of from 300 to 4000, more preferably in the range of from 300 to 2500 and most preferably in the range of from 500 to 2000. Applicants have discovered that if the molecular weight of these reactive oligomers exceeds 4000, these reactive oligomers would become too viscous. As a result, larger amounts of solvent would be needed to produce a coating composition that can be sprayed by conventional spray coating devices. However, such a coating composition will not be a low VOC coating composition. Furthermore, the polydispersity of all of these reactive oligomers does not exceed about 1.7. Preferably, the polydispersity is in the range of from 1.01 to 1.7, more preferably in the range of from 1.01 to 1.5 and most preferably in the range of from 1.01 to 1.3. Applicants have discovered that if the polydispersity of these reactive oligomers exceeds 1.7, a coating composition which includes such a reactive oligomer would produce coating compositions that are too viscous for conventional spray coating devices.
Applicants have unexpectedly discovered that the presence of a linear or branched cycloaliphatic moiety in the silicon/hydroxyl reactive oligomer, silicon reactive oligomer and the silicon free reactive oligomer is critical for solubilizing of these reactive oligomers in a variety of organic solvents described below. The presence of the cycloaliphatic moiety also improves the miscibility of the multiple components of a coating composition and help maintain the film hardness of a coating resulting therefrom under normal use. All of these reactive oligomers are provided with at least one, preferably 1 to 6 and more preferably 1 to 4 cycloaliphatic rings. Some of the suitable cyclic moieties include 4 to 10 carbon atoms. Cyclohexane moiety is most preferred.
The silicon free reactive oligomer of the silicon/hydroxyl component is provided on an average in the range of from 2 to 10, preferably in the range of from 2 to 6 and more preferably in the range of from 2 to 4 with hydroxyl groups, which may be primary, secondary or a combination thereof. The primary hydroxyl group is a hydroxyl group positioned at the terminal end of the reactive oligomer. The higher the number of primary hydroxyl groups on the reactive oligomer, the higher will be its reactivity and the lower will be the cure temperature of the coating composition. Thus, the coating composition containing reactive oligomers provided with one or more primary hydroxyl groups would cure under ambient conditions.
The silicon free reactive oligomer of the present invention is produced by first reacting a multifunctional alcohol having a linear or branched cycloaliphatic moiety, such as, pentaeiythritol, hexandiol, trimethyol propane with alicyclic monomeric anhydrides, such as for example, hexahydrophthalic anhydride or methylhexahydrophthalic anhydride to produce an oligomeric acid. Mixtures of the foregoing anhydrides may also be used. Non-alicyclic anhrydides (linear or aromatic), such as for example, succinic anhydride or phthalic anhydride could also be added to the alicyclic monomeric anhydrides. Oligomeric acids having at least one hydroxyl functionality are also suitable. Such oligomeric acids are prepared. by reacting the multifunctional alcohol with less than a stochiometric amount of the monomeric anhydride.
The oligomeric acid is then reacted with a monofunctional epoxy, at a reaction gage pressure of less than 14 kg/cm2 (200 psig), preferably at the reaction gage pressure in the range of from 0 kg/cm2 to 2.1 kg/cm2 (0 to 30 psig) to produce the reactive oligomer. The oligomerization is generally carried out at a reaction temperature in the range of from 60xc2x0 C. to 200xc2x0 C., preferably in the range of from 80xc2x0 C. to 170xc2x0 C., and more preferably in the range of from 90xc2x0 C. to 150xc2x0 C. Typical reaction time is in the range of from 1 hour to 24 hours, preferably from 1 hour to 4 hours.
The foregoing two-step process ensures that the hydroxyl functionalities are uniformly distributed on each oligomeric chain of the silicon free reactive oligomer.
The monofunctional epoxies suitable for use in the present invention include alkylene oxide of 2 to 12 carbon atoms. Ethylene, propylene and butylene oxides are preferred, ethylene oxide is more preferred. Other epoxies, such as, Cardura(copyright) E-10 glycidyl ester, supplied by Exxon Chemicals, Houston, Tex. may be used in conjunction with the monofunctional epoxies, described above.
Several methods are available for producing the silicon reactive oligomers.
For example, the silicon free reactive oligomers may be reacted with a stoichiometric amount of an isocyanato silane compound to replace all of the hydroxyl groups on the silicon free reactive oligomers with silane functionalities. If less than stoichiometric amount of the isocyanato silane compound is utilized, the resulting reactive oligomer will be the silicon/hydroxyl reactive oligomer having silane and hydroxyl functionalities. If a silicon free reactive oligomer having only two hydroxyl functionalities is used, then at least one of the hydroxyl groups is replaced with a silane functionality.
The foregoing method results in a silicon free reactive oligomer with silane functionalities of the following formula: 
wherein R1 is the remainder portion of the silicon free reactive oligomer, m as stated earlier varies in the range of from 2 to 10, R2 is methyl or ethyl, R3 is an alkyl or cycloalkyl radical having 1 to 10 carbon atoms and n is 0, 1 or 2. Some of the preferred silane compounds include isocyanato propyl trimethoxysilane.
Another suitable method for producing the silicon reactive oligomers having silane functionalities includes reacting the oligomeric acid having cycloaliphatic moiety with a stoichiometric amount of an epoxysilane, such as those supplied by WITCO Corporation of Friendly, W.Va. under the trademark A-186 Silane coupling agent of the formula xcex2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane. A-187 Silane coupling agent of the formula glycidyl propyltrimethoxysilane is also suitable. To prevent gelation. all of the acid groups have to be reacted with glycidylsilane molecule through the glycidyl group.
Still another suitable method for producing the silicon reactive oligomers having silane functionalities includes reacting oligomeric epoxies having a cycloaliphatic moiety with an aminosilane. Some of the suitable epoxies include Araldite(copyright) CY184 epoxy resins of the formula diglycidyl ester of 1,2-cyclohexane diacarboxylic acid supplied by Ciba Specialty Chemicals of Tarrytown, N.Y. and ERL-4221. ERL-4299 and ERL-4206 Cycloaliphatic epoxides supplied by Union Carbide of New York, N.Y. Some of the suitable aminosilanes include A-1100 Silane coupling agent having the formula gamma-arninopropyltriethoxysilane supplied by WITCO Corporation of Friendly, W.Va. A-1110 and A-1170 Silane coupling agents are also suitable.
The following method represents the reaction between the silicon free oligomer having hydroxyl functionalities with a silane compound for producing the silicon reactive oligomers having silicate functionalities: 
wherein R1 is the remainder portion of the silicon free reactive oligomer, m as stated earlier varies in the range of from 2 to 10, R2 is methyl or ethyl, R3 is an alkyl or cycloalkyl radical having 1 to 10 carbon atoms and n is 0, 1 or 2. Some of the preferred silane compounds include tetramethoxysilane and methyl trialkoxysilane, wherein the alkoxy groups contains 1 to 12 carbon atoms. Methyl trimethoxysilane is more preferred.
The silicon reactive oligomers having silane and silicate functionalities may be produced by reacting a polyol with a multifunctional silane.
The suitable polyol include simple diols. triols, and higher hydroxyl alcohols typically having a hydroxyl equivalent weight of about 30 to 1000, preferably from 50 to 500.
The simple diols, triols, and higher hydroxyl alcohols are generally known, examples of which include 2,3-dimethyl-2,3-butanediol (pinacol), 2,2-dimethyl-1-1,3-propanediol (neopentyl glycol), 2-ethyl-2-methyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, 1,12-dodecanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4xe2x80x2-isopropylidenedicyclohexanol, 4,8-bis(hydroxyethyl)tricyclo[5.2.1.0]decane, 1,3,5-tris(hydroxyethyl)cyanuric acid (theic acid), 1,1,1-tris(hydroxymethyl)ethane, glycerol, pentaerythritol, sorbitol, and sucrose.
The multifunctional silanes include but are not limited to 1,2-bis(trimethoxysilyl)ethane, 1,6-bis(trimethoxysilyl)hexane, 1,8-bis(trimethoxysiyl)octane, 1,4-bis(trimethoxysilylethyl)benzene, bis(3-trimethoxysilylpropyl)amine, bis(3-trimethoxysilylpropyl)ethylenediamine, bis(trimethoxysilyl) derivatives of the following polyolefins: limonene and other terpines, 4-vinyl-1-cyclohexene, 5-vinyl-2-norbornene, norbomadiene, dicyclopentadiene, 1,5,9-cyclododecatriene, tris(trimethoxysilyl) derivatives of higher polyolefins, such as 1,2,4-trivinylcyclohexane. Examples of the substituted multifunctional silanes include but are not limited to bis and tris(trimethoxysilane) derivatives of polyunsaturated polyesters of the corresponding acids: trimellitic acid, cyclohexane dicarboxylic acids, 10-undecenoic acid, vinylacetic acid; and bis and tris(trimethoxysilane) derivatives of polyunsaturated polyethers of the corresponding polyols: 1,4-cyclohexanedimethanol, and 4,4xe2x80x2-isopropylidenedicyclohexanol. The multifunctional silane where a diol is reacted with bistrimethoxysilated adduct, described below, is preferred. 
Alternatively, the silicon reactive oligomer and the silicon/hydroxyl reactive oligomer may be prepared by reacting the aforedescribed silane compounds, or a combination thereof with the aforedescribed oligomeric alcohols, which contain cycloaliphatic moiety. Such oligomeric alcohols include cyclohexane dimethanol. The silicon/hydroxyl component of the binder of the present invention may be blended with non-alicyclic (linear or aromatic) oligomers, if desired. Such non-alicyclic-oligomers may be made by the aforedescribed process by using non-alicyclic anhydrides, such as succinic or phthalic anhydrides, or mixtures thereof. Caprolactone oligomers described in the U.S. Pat. No. 5,286,782 may be also used.
The hydroxy acrylic polymer, hydroxy polyester, the silicon free reactive oligomer or a combination thereof is blended in the range of from 0.1 percent to 95 percent, preferably in the range of from 10 percent to 90 percent, more preferably in the range of from 20 percent to 80 percent and most preferably in the range of from 50 percent to 70 percent, all based on the total weight of the silicon/hydroxyl component, with the silicon reactive oligomer of the silicon/hydroxyl component of the binder of the present invention. The hydroxy acrylic polymer and the silicon free reactive oligomer are preferred and the hydroxy acrylic polymer is more preferred. Applicants have discovered that by adding one or more of the foregoing component to the silicon/hydroxyl component, the coating composition resulting therefrom provides the coating with improved appearance, sag resistance, and flow and leveling properties.
If desired, the components described in the foregoing paragraph may be also blended with the silicon/hydroxyl reactive oligomer in the same proportions as those provided in the foregoing paragraph.
The hydroxy acrylic polymer has a GPC weight average molecular weight exceeding 3000, preferably in the range of from 3000 to 20,000, more preferably in the range of 6000 to 20,000, and most preferably in the range of from 8000 to 12,000. The Tg of the hydroxy acrylic polymer varies in the range of from 0xc2x0 C. to 100xc2x0 C., preferably in the range of from 30xc2x0 C. to 80xc2x0 C. The hydroxy acrylic polymer is provided on an average in the range of from 2 to 10, preferably in the range of from 2 to 6 and more preferably in the range of from 2 to 4, with functional groups. Of these functional groups, on an average at least one, preferably in the range of 1 to 4 and more preferably in the range of from 2 to 4 must be hydroxyl groups, the remainder of the groups are silane, silicate or a combination thereof. The foregoing average range may be attained by blendine hydroxy acrylic polymers having various numbers of functional groups.
The hydroxy acrylic polymer suitable for use in the present invention may be any conventional solvent soluble hydroxy acrylic polymer conventionally polymerized from typical monomers, such as alkyl (meth)acrylates having alkyl carbon atoms in the range of from 1 to 18, preferably in the range of from 1 to 12, styrene and hydroxy functional monomers, such as, hydroxy ethyl (meth)acrylates.
The hydroxy acrylic polymer may be reacted with less than stoichiometric amount of the silane compounds (described earlier), or a combination thereof to provide the hydroxy acrylic polymer with hydroxy, silane or silicate functionalities. Alternatively, the hydroxy acrylic polymer may be polymerized by including a monomer mix, silane-functional monomers, which include acrylate alkoxy silanes, such as gamma acryloxypropyltrimethoxy silane; methacrylatoalkoxy silanes, such as garima-methacryloxypropyltrimethoxy silane, gamma trimethoxy silyl propyl methacrylate, and gamma trimethoxy silyl prepyl acrylate, and gamma-methacryloxypropyltris(2-methoxyethoxy) silane; vinylalkoxy silanes, such as vinyltrimethoxy silane, vinyltriethoxy silane and vinyltris(2-methoxyethoxy) silane; vinylacetoxy silanes, such as vinylmethyl diacetoxy silane, acrylatopropyl triacetoxy silane, and methacrylatopropyltriacetoxy silane; and combinations thereof. Gamma-methacryloxypropyltrimethoxy silane is preferred.
The hydroxy polyester suitable for use in the present invention may be a conventional hydroxy polyester having a GPC weight average molecular weight exceeding 1500, preferably in the range of from 1500 to 100,000, more preferably in the range of 2000 to 50,000, still more preferably in the range of 2000 to 8000 and most preferably in the range of from 2000 to 5000. The Tg of the hydroxy polyester varies in the range of from xe2x88x9250xc2x0 C. to +100xc2x0 C., preferably in the range of from xe2x88x9220xc2x0 C. to +50xc2x0 C.
The hydroxy polyester is conventionally polymerized from suitable polyacids, including cycloaliphatic polycarboxylic acids, and suitable polyols, which include polyhydric alcohols. Examples of suitable cycloaliphatic polycarboxylic acids are tetrahydrophthalic acid, hexahydrophthalic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4-methylhexahydrophthalic acid, endomethylenetetrahydrophthalic acid, tricyclodecanedicarboxylic acid, endoethylenehexahydrophthalic acid, camphoric acid, cyclohexanetetracarboxylic acid and cyclobutanetetracarboxylic acid. The cycloaliphatic polycarboxylic acids can be used not only in their cis but also in their trans form and as a mixture thereof. Examples of suitable polycarboxylic acids which, if desired, can be used together with the cycloaliphatic polycarboxylic acids are aromatic and aliphatic polycarboxylic acids, such as, for example, phthalic acid, isophthalic acid, terephthalic acid; halogenophthalic acids, such as, tetrachloro- or tetrabromophthalic acid; adipic acid; glutaric acid; azelaic acid; sebacic acid; furmaric acid; maleic acid; trimellitic acid; and pyromellitic acid.
Suitable polyhydric alcohols include ethylene glycol, propanediols, butanediols, hexanediols, neopentylglycol, diethylene glycol, cyclohexanediol, cyclohexanedimethanol, trimethylpentanediol, ethylbutylpropanediol, ditrimethylolpropane, trimethylolethane, trimethylolpropane, glycerol, pentaerythritol, dipentaerythritol, tris(hydroxyethyl) isocyanate, polyethylene glycol and polypropylene glycol. If desired, monohydric alcohols, such as, for example, butanol, octanol, lauryl alcohol, ethoxylated or propoxylated phenols may be also included along with polyhydric alcohols. The details of the hydroxy polyester suitable for use in the present invention are further provided in the U.S. Pat. No. 5,326,820, which is incorporated herein by reference. One of the commercially available polyester, which is particularly preferred, is SCD(copyright)-1040 polyester, which is supplied by Etna Product Inc., Chagrin Falls, Ohio.
The hydroxy polyester may be reacted with less than stoichiometric amount of the silane compounds (described earlier), or a combination thereof to provide the hydroxy polyester with hydroxy, silane or silicate functionalities.
In addition to the forgoing components, the silicon/hydroxyl component of the binder of the present invention may further contain up to 40 percent, preferably in the range of from 5 percent to 35 percent, more preferably in the range of from 20 percent to 30 percent, all in weight percent based on the total weight of the binder of a dispersed acrylic polymer which is a polymer particle dispersed in an organic media, the polymer particle being emulsion stabilized by what is known as steric stabilization. Preferably, the polymer particle is provided with a core having macromonomer chains or arms attached to it. The preferred average particle size of the core is in the range of from 0.1 micron to 0.5 micron, preferably in the range of from 0.15 micron to 0.4 micron, more preferably in the range of from 0.15 micron to 0.35 micron.
The dispersed acrylic polymer includes in the range of from about 10 percent to 90 percent, preferably in the range of from 50 percent to 80 percent all in weight percent based on the weight of the dispersed polymer, of a core formed from high molecular weight polymer having a weight average molecular weight of 50,000 to 500,000, preferably in the range of from 50,000 to 200,000, more preferably in the range of from 50,000 to 150,000. The arms make up 10 percent to 90 percent, preferably 10 percent to 59 percent, all in weight percent based on the weight of the dispersed polymer. The arms are formed from a low molecular weight polymer having weight average molecular weight in the range of from 1,000 to 30,000, preferably in the range of from 3000 to 20,000, more preferably in the range of from 3000 to 15,000.
The core of the dispersed acrylic polymer includes polymerized acrylic monomer(s) optionally copolymerized with ethylenically unsaturated monomer(s). Suitable monomers include styrene, alkyl (meth)acrylate having alkyl carbon atoms in the range of from 1 to 18, preferably in the range of from 1 to 12; ethylenically unsaturated monocarboxylic acid, such as, (meth)acrylic acid, and silane-containing monomers. Other optional monomers include hydroxyalkyl (meth)acrylate or acrylonitrile. Optionally, the core may be crosslinked through the use of diacrylates or dimethacrylates, such as, allyl methacrylate or through post reaction of hydroxyl moieties with polyfunctional isocyanates.
The macromonomer arms attached to the core may be polymerized from monomers, such as alkyl (meth)acrylates having 1 to 12 carbon atoms. Typical hydroxy-containing monomers are hydroxy alkyl (meth)acrylates, described earlier.
The crosslinking component of the binder includes a blocked crosslinker or an unblocked crosslinker. The crosslinking component, which contains the unblocked crosslinker is stored separately from the silicon/hydroxyl component prior to application, i.e., a two-pack curable coating composition. These components are then mixed just before use. By contrast, the crosslinking component, which contains the blocked crosslinker is stored in the same container with the silicon/hydroxyl component, i.e.; a one-pack curable coating composition.
The unblocked or blocked crosslinker is an oligomeric crosslinker or a blend thereof. The unblocked or blocked crosslinker is provided with at least two isocyanate groups, such that the ratio of equivalents of isocyanate of the unblocked or blocked oligomeric crosslinker per equivalent of the hydroxyl of the silicon/hydroxyl component is in the range of from 0.3/1 to 3.0/1, preferably in the range of from 0.7/1 to 2/1, more preferably in the range of from 0.8/1 to 1.3/1.
Some of suitable unblocked oligomeric crosslinkers include aromatic, aliphatic, or cycloaliphatic isocyanates, triffunctional isocyanates and isocyanate functional adducts of a polyol and difunctional isocyanates. Some of the particular isocyanates include diisocyanates, such as 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, biscyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-phenylene diisocyanate, 1,5-napthalene diisocyanate, bis-(4-isocyanatocyclohexyl)-methane and 4,4xe2x80x2-diisocyanatodiphenyl ether.
Some of the suitable trifunctional isocyanates include triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate, and 2,4,6-toluene triisocyanate. Trimers of diisocyanate, such as the trimer of hexamethylene diisocyante sold under the trademark Desmodur(copyright)N-3390 by Bayer Corporation of Pittsburgh, Pa., and the trimer of isophorone diisocyanate are also suitable. Furthermore, trifunctional adducts of triols and diisocyanates are also suitable. Trimers of diisocyanates are preferred and trimers of isophorone and hexamethylene diisocyantes are more preferred.
The blocked crosslinker has an isocyanate portion and a blocker portion. The isocyanate portion of the blocked crosslinkers are well-known in the art, and include toluene diisocyanates, isocyanurates of toluene diisocyanate, diphenylmethane 4,4xe2x80x2-diisocyanate, isocyanurates of 4,4xe2x80x2-diisocyanate, methylenebis-4,4xe2x80x2-isocyanatocyclohexane, isophorone diisocyanate, isocyanurates of isophorone diisocyanate, 1,6-hexamethylene diisocyanate, isocyanurates of 1,6-hexamethylene diisocyanate, 1,4-cyclohexane diisocyanate, p-phenylene diisocyanate, triphenylmethane 4,4xe2x80x2,4xe2x80x3-triisocyanate, tetramethyl xylene diisocyanate, metaxylene diisocyanate, and polyisocyanates. Groups suitable for use as the blocker portion of the blocked crosslinker are also well-known in the art, and include alcohols, lactarns, oximes, malonic esters, alkylacetoacetates, triazoles, pyrazoles (e.g. dimethyl pyrazole), phenols and anines. Of these, oximes (e.g., acetone oxime, methyl ethyl ketoxime, methylamyl ketoxime) are preferred. Most preferably, the blocked isocyanate is the isocyanurate of 1,6-hexamethylene diisocyanate, wherein the blocker portion is an oxime (e.g., acetone oxime, methylethyl ketoxime, methylamyl ketoxime) or a pyrozole (e.g. dimethyl pyrazole). Some of the commercial examples of blocked isocyanate include BL 3175 MEKO blocked HDI isocyanurate trimer and BL 4165, MEKO blocked IPDI isocyanurate trimer both supplied by Bayer Corporation of Pittsburgh, Pa. Another suitable commercial blocked isocyanate is BI 7982, 3,5-dimethyl pyrazole blocked HDI isocyanurate trimer supplied by Baxenden Chemicals Ltd., Lancashire, England.
The crosslinking component may optionally include in the range of from 0.1 percent to 30 percent, preferably in the range of from 5 percent to 25 percent, more preferably in the range of from 10 percent to 20 percent, all in weight percentages based on the total weight of binder solids, of the following one or more additional crosslinkers.
Aldimine oligomers which are the reaction products of alkyl aldehydes, such as isobutyraldehyde, with diamines, such as isophorone diamine. Ketimine oligomers which are the reaction product of alkyl ketones, such as methyl isobutyl ketone, with diamines, such as 2-methyl pentamethylene diamine. Polyaspartic esters, which are the reaction product of diamines, such as isopherone diamine, with dialkyl maleates, such as diethyl maleate. All of the foregoing additional crosslinkers are well known, including those supplied under the trademark Desmopheno(copyright) amine co-reactants by Bayer Corporation, Pittsburgh, Pa. Melamine-fomaldehyde resins, such as CYMEL(copyright) 300, 303, 350, 1156, 1168 and 325 Resins supplied by Cytec Industries of West Patterson, N.J. are suitable. Epoxies, such as Araldite(copyright) CIY 184 epoxy resins from Ciba Specialty Chemicals of Tarrytown, N.Y. and DCE 358 Epoxy Resin from Dixie Chemicals in Texas.
The crosslinking component of the binder preferably includes a catalytic amount of a catalyst for accelerating the curing process. The catalytic amount depends upon the reactivity of the hydroxyl group of the reactive oligomer present in the silicon/hydroxyl component of the binder. Generally, in the range of 0.001 percent to 5 percent, preferably in the range of from 0.01 percent to 2 percent, more preferably in the range of from 0.02 percent to 1 percent, all in weight percent based on the total weight of binder solids, of the catalyst is utilized. A wide variety of catalysts can be used, such as, tin compounds, including dibutyl tin dilaurate; tertiary amines, such as, triethylenediarnine. These catalysts can be used alone or in conjunction with volatile carboxylic acids, such as acetic acid. Other acid catalysts, such as dodecylbenzene sulfonic acid and phenyl acid phosphate may be also used as catalyst to accelerate cure with the silane compounds. The dodecylbenzene sulfonic acid may be optionally blocked with amines, such as aminomethylpropanol. One of the commercially available catalyst sold under the trademark, Fastcat(copyright) 4202 dibutyl tin dilaurate by Elf-Atochem North America, Inc. Philadelphia, Pa., is particularly suitable.
The coating composition of the present invention, which is formulated into a high solids coating system, further contains at least one organic solvent which is typically selected from the group consisting of aromatic hydrocarbons, such as, petroleum naphtha or xylenes; ketones, such as methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone or acetone. esters, such as butyl acetate or hexyl acetate; and glycol ether esters, such as propylene glycol monomethvl ether acetate. The amount of organic solvent added depends upon the desired solids level as well as the desired amount of the VOC in the composition. The organic solvent may be added to either or both of the components of the binder.
The coating composition of the present invention may also contain conventional additives, such as, pigments, stabilizers, rheology control agents, flow agents, toughening agents and fillers. Such additional additives will, of course, depend on the intended use of the coating composition. Fillers, pigments, and other additives that would adversely effect the clarity of the cured coating will not be included if the composition is intended as a clear coating. The foregoing additives may be added to either the silicon/hydroxyl or crosslinking components, or both, depending upon the intended use of the coating composition. These additives are preferably added to the silicon/hydroxyl component.
The silicon/hydroxyl and crosslinking components, when formulated as a two-pack coating composition for OEM application, are mixed in in-line mixers just prior to use. Alternatively, the components are mixed about 5 to 30 minutes before use to form a pot mix, which has a limited pot life. A layer of the pot mix is typically applied to a substrate by conventional techniques, such as, spraying, electrostatic spraying, roller coating, dipping or brushing. Depending on the type of hydroxyl functionalities included in the silicon/hydroxyl component (primary versus secondary), the layer of the coating composition is then cured under ambient conditions (the silicon/hydroxyl component includes at least one primary hydroxyl functionality) in 30 minutes to 24 hours, preferably in 30 minutes to 3 hours to form a coating on the substrate having the desired coating properties. It is understood that the actual curing time depends upon the thickness of the applied layer and on any additional mechanical aids, such as fans or blowers that provide continuous air flow over the coated substrate to accelerate the cure rate. If desired, the cure rate may be further accelerated by baking the coated substrate at temperatures generally in the range of from 60xc2x0 C. to 150xc2x0 C. for a period of 15 minutes to 90 minutes.
If the silicon/hydroxyl component includes all secondary hydroxyl functionalities, then the layer of the pot mix, as described above, is bake-cured at a bake temperature in the range of from 100xc2x0 C. to 150xc2x0 C. for a period of 90 minutes to 15 minutes. The foregoing baking step is particularly useful under OEM conditions.
A layer of the one-pack coating composition is typically applied to a substrate by conventional techniques, such as, spraying, electrostatic spraying, roller coatings dipping or brushing. The layer of the one-pack coating composition is bake-cured at a bake temperature in the range of from 100xc2x0 C. to 150xc2x0 C. for a period of 90 minutes to 15 minutes. The foregoing baking step is particularly useful under OEM conditions.
The coating compositions of the present invention are particularly useful as a clear coating for outdoor articles, such as automobile and other vehicle body parts. The substrate is generally prepared with a primer and or a color coat or other surface preparation prior to applying the coating of the present composition.